1. Field of the Invention
The present invention relates to a printed wiring board, and in particular, to improvement of a cover lay which is laminated on a surface of a polyimide resin basefilm on which conductive circuits made from copper foil are formed.
2. Description of the Related Art
In a conventional cover lay which is used for a printed wiring board, the cover lay is made by applying an adhesive such as epoxy resin, acrylic resin, etc., to a surface of a sheet made from polyimide resin. The printed wiring board on which such a cover lay is laminated has flexibility. A rigid flex printed wiring board is made by laminating rigid board both surfaces of which are covered with conductive circuits onto some parts of the printed wiring board which has flexibility.
Namely, in a conventional rigid flex printed wiring board, a surface of the flex part is covered with cover lay made from polyimide resin which has adhesive applied thereto. However, a part of the cover lay can easily exfoliate from the bonding layer made from epoxy resin and can blister, since cover lay made from polyimide resin has hygroscopicity. Accordingly, such a conventional rigid flex printed wiring board would have an inferior appearance and would be disadvantageous. Further, the possibility of losing electric characteristics occurs, because water can enter the portion at which the cover lay exfoliates from the bonding layer.